Fossils and Fossilization
A fossil is the remains of an ancient life form—plant or animal—or its traces, such as nesting grounds, footprints, worm trails, or the impressions left by leaves, preserved in rock. Fossil traces are called ichnofossils. Fossilization refers to the series of postmortem changes that lead to replacement of minerals in the original hard parts (shell, skeleton, teeth, horn, scale) with different minerals, a process known as remineralization. Infrequently, soft parts may also be mineralized and preserved as fossils. A new category of subfossil—a fossil that has not yet begun to mineralize—is increasingly recognized in the scientific literature. Many subfossils originated in the Holocene or Recent, the period that we now live in, and cannot be dated with any greater accuracy. In addition, the term "fossil" is applied in other ways, for example, to preserved soils and landscapes such as fossil dunes. Through the study of fossils, it is possible to reconstruct ancient communities of living organisms and to trace the evolution of species.
Fossils occur on every continent and on the sea floors. The bulk of them are invertebrates with hard parts (for example, mussels). Vertebrates, the class that includes reptiles (for example, dinosaurs) and mammals (mastodons, humans), are a relatively late development, and the finding of a large, complete vertebrate fossil, with all its parts close together, is rare.
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